The present invention relates to a swash plate type compressor having a single-headed piston for use in, for example, a vehicle air conditioner.
In a variable displacement swash plate type compressor shown in FIG. 9, in general, a compressor housing is formed such that a front housing 102 and a rear housing 103 are arranged to sandwich a cylinder block 101. A crank chamber 104 is formed between the front housing 102 and the cylinder block 101. A drive shaft 105 across the crank chamber 104 is rotatably supported by the housing. A first end of the drive shaft 105 penetrates through a through hole 106 of the front housing 102, whereas a second end of the drive shaft 105 is in the crank chamber 104. A shaft seal 107 is arranged to seal a gap between the drive shaft 105 and the front housing 102, thereby preventing refrigerant in the crank chamber 104 from leaking out. A plurality of cylinder bores 108 are formed in the cylinder block 101 to surround the drive shaft 105. A piston 109 is disposed in each of the cylinder bores 108 and reciprocates there. A suction chamber 110 and a discharge chamber 111 are formed in the rear housing 103.
A swash plate 113 is mounted on the drive shaft 105 through a hinge mechanism 112 and rotates together with the drive shaft 105. The swash plate 113 is capable of sliding in the axial direction of the drive shaft 105 and of inclining with respect to the drive shaft 105. Each piston 109 is engaged with an outer peripheral portion of the swash plate 113 through a pair of shoes 114 so that the rotational movement of the drive shaft 105 is converted to the reciprocating movement of the piston 109. Refrigerant in the suction chamber 110 is drawn into the cylinder bore 108 and compressed there by the reciprocating piston 109. When pressure in the crank chamber 104 is adjusted, an inclination angle of the swash plate 113 changes. Therefore, the piston stroke changes. Accordingly, the discharge capacity of the compressor becomes variable. For example, the inclination angle of the swash plate 113, the angle between a plane perpendicular to the drive shaft 105 and the swash plate 113, decreases when the pressure in the crank chamber 104 increases. Reduction of the piston stroke decreases the discharge capacity of the compressor.
During operation of the compressor, compressive reaction force of each piston 109 acts on the drive shaft 105 through the swash plate 113. On the other hand, pressure difference between the pressure Pc in the crank chamber 104 and the atmospheric pressure P0, which is multiplied by a cross-sectional area of the drive shaft 105 substantially at which the shaft seal 107 is provided, acts on the drive shaft 105. Both the reaction force and the pressure difference intend to push the drive shaft 105 frontwards. The thrust load based on the reaction force and the pressure difference is supported by the front housing 102 through a thrust bearing 116 arranged between a rotor 115 or lug plate and the front housing 102.
In recent years, a compressor has been proposed for use in a refrigerant circuit which employs a refrigerant gas such as carbon dioxide, instead of chloro-fluoro carbon. Such a circuit, after compression of the gas, cools down the gas in a super critical range that exceeds a critical temperature of the gas. For example, according to Japanese Patent Application Publication No. 11-223179 discloses a variable displacement type of compressor employing carbon dioxide as refrigerant. In this compressor, refrigerant in a discharge pressure region supplied into the crank chamber 104 is controlled by an electric displacement control valve 117 as shown conventionally in FIG. 9. The amount of refrigerant passing through the refrigerant circuit is adjusted based on the external data such as a heat load.
When the circuit employs chloro-fluoro carbon as refrigerant, the pressure Pc in the crank chamber is relatively small, less than or equal to 9.8xc3x97105 Pa. However, when the refrigerant such as carbon dioxide is employed, the pressure Pc in the crank chamber rises greatly. For example, employment of carbon dioxide raises the pressure Pc higher than the pressure in employment of chloro-fluoro carbon by about several tens to a hundred xc3x97104 Pa. As a result, the thrust load supported by the thrust bearing 116 increases greatly, and sealing function of the shaft seal 107 against the high pressure is required.
When the thrust load acting on the drive shaft 105 in the same direction as the compressive reaction force becomes higher, mechanical loss increases as well as the power consumption to drive the drive shaft 105. The power consumption is typically apparent when the power of the drive source such as an engine is transmitted to the drive shaft 105 without using a clutch, for instance, in a clutchless variable displacement type of swash plate compressor. That is, when the compressor is driven in a minimum capacity state or off-drive state, the power consumption, which should be minimum, increases.
Further, when the shaft seal 107 is arranged in the crank chamber region, the lubrication of the shaft seal 107 is not satisfactorily performed because refrigerant in the crank chamber has not only high pressure but high temperature.
Accordingly, it is a first object of the present invention to provide a swash plate type compressor in which required power to drive the compressor is reduced by reducing a thrust load in the same direction as compressive reaction force acting on a drive shaft.
To achieve the above first object, a swash plate type compressor of the present invention has a housing including a suction chamber, a discharge chamber and a crank chamber, a drive shaft rotatably supported by the housing, the drive shaft having a first end protruding from the housing and a second end disposed in the crank chamber, a cylinder bore defined between the crank chamber and the first end of the drive shaft, a single-headed piston disposed in the cylinder bore to be reciprocated, and a cam plate rotatably mounted on the drive shaft in the crank chamber, the cam plate being operatively engaged with the piston, whereby rotational movement of the drive shaft is converted to reciprocating movement of the piston through the cam plate.
In the present invention, when refrigerant is compressed during operation of the compressor, the compressive reaction force of the piston acts on the drive shaft through the cam plate thereby pushing the drive shaft toward its second end. On the other hand, pressure in the crank chamber acts on the second end portion of the drive shaft against atmospheric pressure acting on the first end of the drive shaft so that pressure difference between them pushes the drive shaft in the opposite direction to the reaction force. Therefore, according to the present invention the power to drive the drive shaft of the compressor is reduced by reduction of thrust force acting on the drive shaft.
It is a second object of the present invention to provide a swash plate type compressor in which a shaft seal arranged to seal a gap between a drive shaft and a housing is improved.
To achieve the above second object according to the present invention, the suction chamber is in the housing defined adjacent to the first end of the drive shaft. The drive shaft is arranged in the housing such that the first end of the drive shaft penetrates the suction chamber and protrudes from the housing. A shaft seal is arranged between the suction chamber and the first end of the drive shaft, thereby sealing the suction chamber.
The foregoing shaft seal arrangement of the present invention simply requires resistance against pressure difference between atmospheric pressure and suction pressure which is lowest in the compressor. Accordingly, durability of the shaft seal is sufficiently extended, and sealing function thereof is improved. This is apparently effective when carbon dioxide and the like is employed as refrigerant instead of chloro-fluoro carbon, because carbon dioxide is used in its high pressure range, super critical range. The pressure in the crank chamber of the variable displacement compressor is to be higher than that of the fixed displacement compressor. Accordingly, the variable displacement compressor according to the present invention is more effective than the fixed displacement compressor according to the present invention because carbon dioxide is used in its high pressure range, super critical range.